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Design                                                                                                 Design















 Table 1: Summary of Effect of Switching Frequency Increasing on BW/PM.
 the requirement set by Equation (10). Once again,   MPQ4430 BODE PLOT AT
 since the switching frequency is configurable in   DIFFERENT SWITCHING
 this part, the maximum configurable switching   FREQUENCIES

 frequency is considered in Equation (10). Consider-  Figure 4 shows the bode measurement of the
 ing the maximum switching frequency of 2.5MHz,   MPQ4430 as the switching frequency — and con-
 the target COMP-P frequency (f COMP-P ), must be set   sequently the inductor value — change.
 close to f  / 2 = 2.5MHz / 2 = 1.25MHz.
 SW
 There are a couple of important observations;
 For this part, R COMP  and C  are 460kΩ and 0.2pF,   Table 1 summarizes the results.
 HF
 respectively. This results in f COMP-P  being 1.7MHz,

 which is close enough to the 1.25MHz target.   First of all, as seen in the bode measurements, at   Figure 6: Bode Diagram of MPQ4430 at 2.5MHz With and Without Feed-Forward Capacitor.
 higher switching frequencies the PM is improved.
 Lastly, C  is compared with C COMP  to ensure the   This is because, based on Equation (2), the pole   switching frequencies is by adding a feed-for-  CONCLUSION
 HF
 requirement set by Equation (11) is met. In this   due to the inductor (ω ) is pushed further out as   ward capacitor (C ) to the feedback network (see   This article presented a systematic approach
 L
                           FF
 part, C  and C COMP  are 0.2pF and 52pF, respective-  the switching frequency increases. This causes   Figure 5).   to evaluate the capability of the internal com-
 HF
 ly. Therefore, C  is approximately 0.3% of C COMP .   less negative phase at the BW frequency and an   pensation networks based on the application
 HF
 So, the requirement for the C  vs. C COMP  value (C    increase in the PM, which further confirms that   Adding a feed-forward capacitor can great-  switching frequency. The proposed evaluation
 HF
 HF
 < 4% x C COMP ) is also met.  setting the COMP-Z frequency based on the min-  ly improve the BW and PM of the system. The   technique involved three basic checks to ensure
 imum switching frequency is a   frequency response of the MPQ4430 was taken   that the internal compensation network is prop-
 wise decision.  at a 2.5MHz switching frequency, both with and   erly designed for an application with a known
          without a 20pF feed-forward capacitor. With the      or configurable switching frequency. In certain
 Note that, since R COMP  and C COMP    additional capacitor, BW and PM improved.  cases, adding external knobs can further improve
 are fixed, and the increase in                                the transient performance of the system. These
 the switching frequency only                                  principles were applied to the MPQ4430, which
 affects the second pole in the                                verified the effectiveness of the technique.
 power stage, the BW is rela-

 tively fixed. Therefore, it may
 be desirable to increase the
 BW as the switching frequency   For More Information
 increases. This can be accom-
 plished by adding external     ▶ Monolithic Power Systems
 knobs.


                 ▶ Buck Regulators
 An effective way to increase
 Figure 5: Adding a Feed-Forward Capacitor to the Compensation Network to Improve
 Transient Response.  the loop BW and PM at higher

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